Attempting to install a EUR v1.5 update on a USA base game will either fail to install or cause a crash on launch. Always verify your Title ID via FBI (on 3DS) or your ROM properties (on emulator).
In the case of Pokémon X, the game is in terms of internal language support. Whether you have the USA or EUR update file, you can still play the game in English, French, German, Italian, Japanese, Korean, or Spanish. However, for the update to work, it must match the region of your base game . pokemon x update 15 decrypted 3ds eur usa top
If you own the game and want to play it in a modern, "top-quality" way, here is the standard setup: Attempting to install a EUR v1
Here is a breakdown of what this update fixed and how you can get it running. What’s New in Version 1.5? Whether you have the USA or EUR update